The present invention relates to a sensor element, particularly a lambda probe for analyzing exhaust gases of internal combustion engines, having a protective device.
The sensitive component of customary lambda probes, as are often used for analyzing exhaust gases in the exhaust system of internal combustion engines, is usually surrounded by a protective sleeve to protect against damage as a result of mechanical and thermal influences; the protective sleeve allows the admission of the exhaust gas to be analyzed through suitable openings to the sensitive component of the lambda probe arranged within the protective sleeve. This protective sleeve is used first of all to avoid transport and installation damage, as well as to avoid thermal shock stress of the actual sensitive, heated lambda-probe component due to contact with drops of water.
Such a protective sleeve represents a compromise between sufficient protective action, especially against water admission during cold starts in the following warm-up phase of the engine, and the lowest possible reaction time of the measuring signal of the sensor element in response to a change in the oxygen content in the exhaust gas.
In order not to reduce the reaction time of the sensor element too much, e.g. by a design of the protective sleeve with very high protective action, under the state of the art the protective sleeve is designed such that in the warm-up phase of the engine, action of water or water drops on the sensitive component of the sensor element cannot initially be ruled out completely. At the same time, however, until the dew-point temperature of water in the applied exhaust gas is reached by increasing warm-up of the engine, the heating of the sensitive component to its optimal operating temperature is at first reduced through a change in the heating power to the extent that the temperature of the sensitive component, designed, for example, as a ceramic platelet, does not exceed a temperature of approximately 400xc2x0 C. Only when the warm-up phase of the engine has terminated or has advanced so far that the temperature of the applied exhaust gas has exceeded the dew-point temperature of water is the heating power increased to the extent that the sensitive component reaches its operating temperature of approximately 750xc2x0 C.
It may be that this procedure leads to a good reaction time of the lambda probe, however, it has the disadvantage that in the warm-up phase of the engine, because of the initially lowered operating temperature, the lambda probe is not yet ready for measurements, i.e. operates inadequately for a steady control.
In addition, in known methods heretofore, when the exhaust-gas temperature has exceeded the dew-point temperature of water, i.e. after successful warm-up of the engine, the protection of the sensitive component of the gas sensor implemented up to now by the protective sleeve is no longer necessary to the degree as during the warm-up phase of the engine. The protective sleeve then merely increases the time constants of the lambda probe in an unnecessary manner.
The object of the present invention was is to improve the reaction time of the sensor element in all operating states, accompanied at the same time by sufficient protective action, especially against water admission in the region of the sensitive component of the sensor element.
Compared to the related art, the sensor element of the present invention has the advantage that the admission of gas to the sensitive component is adapted to the specific operating parameters of the sensor element and in particular to the temperature of the applied gas, so that, for example, when used in a lambda probe, the protection of the actual sensitive component is improved by the protective device in the warm-up phase of an internal combustion engine, and when the engine is at operating temperature, the reaction time of the sensor element is reduced, leading to functional advantages in the engine management.
It is particularly advantageous that a software, used frequently till now, for modeling the time-dependent exhaust-gas temperature in the engine management or the heating may be omitted or markedly reduced with respect to its scope.
Thus, it is particularly advantageous if the admission of gas to the sensitive component is regulated as a function of temperature, so that the admittance of gas to the sensitive component is enabled or facilitated at or above a first temperature, while the gas admission is prevented or hindered at or below a second temperature which is lower compared to the first temperature.
It is also advantageous if the protective device is a customary protective sleeve or a protective cap provided with a bimetal region. This bimetal region is preferably a bimetal strip or a bimetal flap, inserted into the protective sleeve or the protective cap or joined to it, with which the size of the gas-admission openings in the protective device can be changed as a function of temperature, and consequently the admission of gas to the sensitive component can be regulated,
In the same way, it is advantageous if the protective device has a bimetal bellows, a part of such a bellows or an expansion structure that is optionally joined to a supporting body and the protective device, and via which the admission of gas to the sensitive component is able to be regulated
The gas-admission openings in the protective device, which are variable as a function of temperature, are advantageously designed so that the sensitive component of the sensor element is optimally protected against water admission at exhaust-gas temperatures below the dew point temperature of water, but at higher temperatures the most optimal exhaust-gas accessibility possible is provided. In this way, due to the optimal protection against water admission, the sensitive component can already be heated to the necessary operating temperature even before the dew point temperature of the applied gas has been reached, so that the functioning of the sensor element is continually fulfilled even at low temperatures, i.e. in the warm-up phase of the engine.